They come from the sun, from exploding stars, from the creation of the universe. Trillions of them pass, like ghosts, through your body every second, leaving no trace. One second later, they have travelled beyond the orbit of the moon, many moving at nearly light speed through the entire Earth.
I’m not making this up. Physicists hypothesized neutrinos in 1930 to resolve a discrepancy between the theory and observation of certain radioactive atoms. Scientists first detected this elusive quantum (or “particle”) in 1956 when they figured out how to detect a few of the neutrinos streaming out of South Carolina’s Savannah River nuclear reactor.
Now Takaaki Kajita and Arthur McDonald have received the 2015 Nobel Prize in physics for the discovery of “neutrino oscillations,” showing that neutrinos have mass and providing a clue about how to extend our most fundamental physical theory.
Some may wonder why this is important. Who cares if neutrinos oscillate? It’s important because an attitude of wonder about the universe lies within all of us. Humans have always had their eyes on the stars, their minds on what it all might mean. Such wondering about our home in the universe leads to the arts and to the sciences, and is precisely what it means to be human. And physicists’ recent wonderings about neutrinos is about as fundamental as it gets, as I hope to show below. Furthermore, the wondering of scientists leads to the technology that provides most of the world’s goods and services, and that makes joyful lives possible while preventing our existence from being brutish and short. And science’s basis in rationality provides a needed model at this time when radically irrational ideologies are tearing much of the world apart.
Kajita headed a team working in 2001 at the Super-Kamiokande neutrino detector in Japan. Imagine a spherical cavern 40 meters across, buried one kilometer underground to screen out background noise, holding 50,000 tons of ultra-pure water, and lined with 13,000 light sensors. Neutrinos seldom interact with anything because they experience only the “weak nuclear force” and gravity. Since gravity is nearly undetectable at the microscopic level, neutrinos effectively experience only the weak force. The weakness of this force explains why neutrinos can plow through our planet without feeling a thing, and why a giant detector is needed to discover any sign of them. You might say neutrinos barely exist.
We’ve known for decades that there are three types of neutrinos. An important question is: Do these things possess mass (or “weight”), or are they massless like the photons of which light is made? It’s important: If neutrinos are massless then they must move at light speed; Einstein’s relativity then tells us that a neutrino’s time stands still, its “clocks cannot tick,” and so it is totally unable to change. But if neutrinos possess even the tiniest mass, quantum physics predicts they not only can but must frequently change their identity by randomly transforming (“oscillating”) from one type to another many times every second. By conferring mass, the universe confers time, change and mortality on neutrinos.
Kajita’s team detected neutrinos created by processes occurring in Earth’s atmosphere. They compared neutrinos coming from above their detector with other neutrinos coming from below their detector after having been created in the atmosphere on the far side of Earth and passing through the entire planet. The team found that the neutrinos from above differed from the neutrinos from below, because the ones from below had had time to partly transform into other types of neutrinos. Such transformations showed that neutrinos must have mass.
McDonald’s team at the Sudbury Neutrino Observatory in Canada clinched this argument in 2001 by detecting neutrinos arriving from the sun, and showing that they also oscillate. The data implies a neutrino mass around one-millionth of the electron’s mass — small indeed.
Our most powerful theory of the high-energy microworld predicts neutrinos have no mass. Because Kajita and McDonald’s discovery represents a crack in this theory, it may point the way toward resolving some cosmic mysteries such as this one: The “normal” matter all around us comprises only 5 percent of the universe. The other 95 percent is “dark matter” and “dark energy,” and we have little idea of what these are. Current physics cannot account for them.
Kajita and McDonald are pioneers in the tradition of the ancient people who built Stonehenge in England, and other more prehistoric two-footed primates who must have peered upward at the starry sky and wondered.
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Art Hobson is a professor emeritus of physics at the University of Arkansas and author of the upcoming Oxford University Press book titled Tales of the Quantum. Email him at [email protected].